The concept of macro is obviously just a concept.
Wikepedia states
Macro photography (or photomacrography[1] or macrography,[2] and sometimes macrophotography[3]), is extreme close-up photography, usually of very small subjects and living organisms like insects, in which the size of the subject in the photograph is greater than life size (though macrophotography technically refers to the art of making very large photographs).[2][4] By some definitions, a macro photograph is one in which the size of the subject on the negative or image sensor is life size or greater.[5] However, in other uses it refers to a finished photograph of a subject at greater than life size.[6]
I personally like 'greater than life size'. This allows more people to participate, as gear is expensive